C-HALOPHOSPHAALKENES - PROBING THE RANGE OF STABILITY AND REACTIVITY TOWARDS BROMINE

The importance of steric protection for the stability of phosphaalkene s RP=CI2 (6) was investigated by varying the size of group R. The phos phaalkene IsP=CI2 (6b) (Is = 2,4,6-triisopropylphenyl) could be prepar ed in 15% isolated yield by reaction of IsPCl2 and HCI3 with two equiv alents of lithium diisopropylamide, in analogy to the synthesis of the stable, sterically more protected, Mes P = CI2 (6a) (Mes* = 2,4,6-tr i-tert-butylphenyl). If the steric protection on the phosphorus was de creased further (R = Es = 2,4,6-triethylphenyl, R = Mes = 2,4,6-trimet hylphenyl), the substitution products RP(CI)N(Pr(i))2 (R = Es (9c) or Mes (9d)) were formed as main products, in addition to thermally unsta ble phosphaalkenes EsP=CI2 (6c) and MeSP=CI2 (6d). The structures of 9 c-d were corroborated by independent synthesis from RPCl2 and two equi valents of diisopropylamine. The reaction of 6a with bromine gave an E /Z mixture of the C-bromo-C-iodophosphaalkene (EZ)-MesP = CEH (E/Z-10 ). Further reaction with bromine proceeded via MesP=CBr2 (5a) and fin ally led to MesP(Br)(CHBr2) (12).